Field of the Invention
The present invention relates to a communication system and, more particularly, to a radio communication system based on the code division multiple access system.
A radio communication system employing the code division multiple access method has characteristics suitable for mobile communication, i.e., its strength against mutual interference with other systems, its adaptability to a multipath, capability of easy hand-off, excellent secrecy and so on. However, a communication system based on the code division multiple access method is different from other multiple access methods in that a plurality of terminal users existing within the same base station cell simultaneously communicate using the same frequency and the same frequency band. Therefore, a communication system based on the code division multiple access method has the following problem in utilization of a channel. Specifically, the problem is that utilization of a channel can be reduced by interference that occurs when a strong communication signal from a user closer to a base station overlaps another weaker signal in the existence of the far-and-near problem, and interference by one station with another station that is caused by unnecessary transmission power from the first station and so on.
Measures to prevent any reduction of utilization of a channel includes the use of a wide band gain through expansion of the band of a signal, the use of a series of codes having little correlation and the use of power control for solving the far-and-near problem. In particular, power control is effective not only in suppressing unnecessary transmission from a station to limit the transmission power of the station to a required minimum thereby minimizing interference of the station with another station but also in solving the far-and-near problem.
However, even when those measures are taken, it is still difficult to prevent any reduction of the quality of communication resulting from an increase in mutual correlation interference (a reduction in a signal-to-noise ratio) attributable to an increase in the number of users who communicate simultaneously.
An effective measure to solve the above-described problem is interference canceling techniques for eliminating interference from another channel, and various types of interference canceling techniques are being studied. A receiver can predict interference of another user, and the receiver can perform a process to eliminate the interference. Those proposed interference canceling systems include an interference canceling system based on a multi-user reception method which utilizes information of a plurality of user signals to be demodulated from a plurality of users. An interference canceling apparatus utilizing this system performs a demodulation process on signals to be demodulated from a plurality of users, performs a re-spreading process and a synthetic process on the demodulated signals to create replica signals, and performs a subtraction process between the replica signals and multi-wave reception signals to realize a demodulation process which suppresses interference components. The effect of suppressing interference components is improved by performing the interference canceling process using a cascade-connected multi-stage configuration.
For information on interference cancellation of this type, for example, the techniques disclosed in Japanese patent Laid-Open No. 51353/1998 and 190494/1998 are well known.